Commit b193351a99 caused the HTML comments to flash when changing
the text cursor. Also, when double-clicking on a comment, the selection
started from the beginning of the file instead.
The following message was displaying when `TOKENIZER_TRACE_DEBUG`
was enabled:
(Tokenizer::nth_last_position) Invalid position requested: 4th-last
of 4. Returning (0-0).
Changing the `nth_last_position` to 3 fixes this. I'm guessing that's
because the parser is at that moment on the second hyphen of the `<!--`
string, so it has to go back only by three characters.
fb5e2670d6